Many examples of magneto-optical recording media having been practically used are formed by accumulating a recording layer, a protective layer and the like on a transparent substrate, and exert recordation and reproduction of information with light incident on the side of the substrate.
Miniaturization of a track width and a recordation mark size is exemplified as means for increasing the recording density of the magneto-optical recording medium, and for example, reduction of a spot size of a light beam for irradiating the magneto-optical recording medium has been practiced. The numerical aperture NA of an objective lens currently used for the magneto-optical recording medium is about from 0.55 to 0.6.
In general, a relational equation φ=λ/2NA holds, in which φ represents a spot size, NA represents a numerical aperture of an objective lens, and λ represents a wavelength of laser light. According to the equation, in order to decrease the spot size φ, it is necessary to increase the numerical aperture NA of the objective lens. In the case where the NA is increased, the resolving power is improved, but the focal length is shortened.
Therefore, in the case where the numerical aperture NA becomes larger, the aberration due to unevenness of the thickness and tilt of the substrate is increased, and thus, it is necessary to decrease the thickness of the substrate as far as possible. Accordingly, it is preferred for realizing high density recordation that recordation and reproduction are conducted with light incident on the side of the recording film, rather than the conventional recordation and reproduction conducted with light incident on the side of the substrate.
Hereinafter, the system conducting recordation and reproduction with light incident on the side of the recording film is referred to as a front illumination system.
In the front illumination system, an optical flying head having a structure, in which a coil generating a magnetic field and an objective lens are integrated, and a proximity actuator are used.
In a magneto-optical recording medium, to which a high recording magnetic field (for example, 300 Oe or more) is necessarily applied, it is generally necessary that a large electric current (about 900 mA) is applied to the coil of the proximity actuator.
In order to improve the rate of recordation and reproduction, on the other hand, it is necessary that the electric current is switched at a high speed, and thus, a high speed transient response is demanded.
In the case where a large electric current is applied, however, the transient response time of the electric current is necessarily large, and thus, high speed switching of the electric current, i.e., high speed switching of the magnetic field, becomes difficult.
Furthermore, upon conducting high speed switching of the electric current, the electric current flows on the surface layer of the coil, and the resistance of the coil is increased to cause heat generation of the coil, which brings about problems in deterioration of performance, service life and the like.
Therefore, high speed switching of an electric current is difficult in a medium requiring a high recording magnetic field, and in order to realize a recording speed of 100 Mbps, it is necessary to suppress the upper limit of the intensity of the recording magnetic field to about 200 Oe or less. In other words, it is necessary to suppress the electric field intensity for satisfying the demanded specification of 100 Mbps.
In order to maintain the recording performance equivalent to that with an externally applied magnetic field of 300 Oe even though the magnetic field intensity is reduced to 200 Oe, it has been practiced that a soft magnetic layer, such as NiFe and the like, is formed as an underlayer of the recording layer on the substrate.
This is because upon applying a magnetic field to a soft magnetic layer, a magnetization state equivalent to that obtained by applying a high magnetic field owing to such a nature that the magnetic flux is not diffused but is concentrated thereto. Therefore, in the case where a soft magnetic layer is provided, recordation and reproduction of a medium requiring a high recording magnetic field can be conducted with a decreased electric current flowing in a coil.
It is necessary that the thickness of the soft magnetic layer is 100 nm or more for obtaining an effect equivalent to that obtained by applying a high magnetic field of 300 Oe.
A conventional ordinary magneto-optical recording medium has a servo pattern having a concavoconvex shape containing lands and grooves on the surface thereof, and a soft magnetic layer is formed on the servo pattern.
In the case where a soft magnetic layer having a thickness of 100 nm or more is formed, however, the lands are broadened to change the width ratio of the lands and the grooves, the taper angle of the grooves becomes significantly obtuse, the edge parts of the lands and the grooves are notably rounded.
Accordingly, a recording layer is thus formed on the soft magnetic layer, to which the servo pattern of the substrate is not accurately reflected, and therefore, recordation and reproduction cannot be conducted with satisfying the demanded performance.
Japanese Unexamined Patent Publication No. Hei 03-105741 (1991) discloses a substrate for a magneto-optical recording medium improved in C/N by providing a soft magnetic material layer having a thickness of about 100 nm or more on an aluminum substrate, and further providing a groove layer thereon.
However, an aluminum substrate generally has a low rigidity, and there is a possibility that it is in contact with an objective lens upon disposing closely thereto, and thus, a proximity actuator is not preferably used.
Furthermore, in the case where the groove layer is provided directly on the soft magnetic layer, the soft magnetic layer is exposed to the air to bring about such a possibility that the magnetic characteristics are changed due to oxidation and nitriding.